It is known in the art that multiple control systems for a vehicle are connected with each other to perform a coordinated control for the vehicle, in which multiple control systems operate in cooperation with each other. The coordinated control for the vehicle between an electrical power steering system and an anti-braking system (ABS), or between the electrical power steering system and an automatic braking system, is disclosed in Japanese Patent Publications H6-56015 or H11-198845.
In the electrical power steering system, in which a steering assist torque is applied to a steering device by an electric motor, a driving current to the electric motor is limited (or the driving current is cut off) in some cases, in order that an overheat of the electric motor or a control unit for the electric motor is prevented, or due to a decrease of power voltage to be applied to the electric motor. In such prior art systems, however, a command signal (command value) outputted by an external control unit (a control unit for a vehicle movement total control) for a target steering angle, a target tire steering angle or a target power assist torque is independently calculated from the conditions of the electrical power steering device. And thereby a desired movement of the vehicle can not be realized in the case that the driving current to the electric motor is limited.
Namely, even in the case that the electrical power steering system is controlled by the command signal from the vehicle movement total control unit, a deviation may occur between the command signal (the command value) for the target steering angle, the target tire steering angle or the target power assist torque and an actual control value from the control unit for the electrical power steering system, when the driving current to the electric motor is limited to prevent the overheat of the electric motor or power devices for the motor control or due to the decrease of the power voltage to the electric motor. As a result, the desired movement of the vehicle can not be obtained.
When the driving current to the electric motor is limited, information of such an operational condition of the current limit is transmitted to other control units (including the vehicle movement total control ECU). In some of prior art systems, the other control units (including the vehicle movement total control unit) stop a transmission of a command signal or any other signals to the control unit for the electrical power steering system, when the other control units receive the signal (information) for the current limit operation. With such prior art systems, the desired movement of the vehicle can not be realized.